Vigorous research is being conducted on organic thin-film transistors that employ organic semiconductor materials. Advantages of using an organic semiconductor material in a transistor include flexibility, areal increase, process simplification due to a simple layer structure, and inexpensive manufacturing equipment.
Organic thin-film transistors can be also manufactured so inexpensively by a print method as to be beyond comparison with conventional Si-based semiconductor devices. Thin-films or circuits can also be easily formed using a print method, a spin-coating method, or a dipping method, for example.
One of the parameters that indicate the characteristics of such an organic thin-film transistor is the current on/off ratio (Ion/Ioff). In an organic thin-film transistor, a current Ids that flows between the source and drain electrodes in a saturation region is expressed byIds=μCinW(VG−VTH)2/2L  (Equation 1)where μ is field-effect mobility, Cin is the capacitance per unit area of the gate insulating film, W is the channel width, L is the channel length, VG is the gate voltage, and VTH is the threshold voltage. Cin is expressed byCin=εε0/d where ε is the relative permittivity of the gate insulating film, ε0 is the permittivity of vacuum, and d is the thickness of the gate insulating film.
It can be seen from Equation (1) that in order to increase the on-current, it is effective to:
1. Improve the field-effect mobility;
2. Reduce the channel length; or
3. Increase the channel width, for example.
The field-effect mobility largely depends on material characteristics, and material development is underway to improve mobility.
On the other hand the channel length is determined by the device structure, and attempts are being made to improve the on-current by devising various device structures. Generally, the distance between source and drain electrodes is reduced to reduce the channel length.
Since organic semiconductor materials do not have a large field-effect mobility, the channel length is usually made 10 μm or smaller and preferably 5 μm or smaller. One method of accurately reducing the source-drain electrode distance is photolithography used in Si process. This process consists of the following steps:
1. Coat a substrate having a thin-film layer with a photoresist layer (resist coating).
2. Remove solvent by heating (prebaking).
3. Irradiate UV light via a hard mask that is patterned in accordance with pattern data, using a laser or an electron beam (exposure).
4. Remove the resist at an exposed portion with an alkali solution (development).
5. Cure the resist at an unexposed portion (a pattern portion) by heating (post baking).
6. Remove the thin-film layer where there is no resist by dipping in an etching solution or exposing to an etching gas (etching).
7. Remove the resist with an alkali solution or oxygen radical (resist stripping).
The above process is repeated after the formation of each thin-film layer to complete an active element. The process, however, requires expensive equipment and takes a long time, thus increasing cost.
In order to reduce manufacturing cost, attempts have been made to form an electrode pattern by a print method, such as an ink-jet print method. Ink-jet print method enables direct drawing of an electrode pattern, whereby the material-use ratio can be increased and, possibly, the manufacturing process can be simplified and cost reduction can be achieved. However, given that it is difficult to reduce the ink discharge quantity with the ink-jet print method, and in the light of landing accuracy which depends on mechanical errors or the like, it is difficult to form a pattern of 30 μm or smaller. Thus, it has been difficult to fabricate a very fine-resolution device using the ink-jet print method alone. Therefore, some technique must be devised in order to fabricate a very fine-resolution device. One example of such a technique involves manipulating the surface on which ink is caused to land.
Patent Document 1 discloses a method that utilizes a gate insulating film formed of a material whose critical surface tension (also referred to as “surface free energy”) is changed by supplying energy such as UV light. Specifically, a site on an insulating film surface where an electrode is to be fabricated is irradiated with UV light, so as to form a region with an increased surface free energy. Then, an aqueous ink that contains an electrode material is applied by an ink jet method, whereby an electrode is formed in the region with increased surface free energy. In this way, a very fine-resolution electrode pattern can be formed on a gate insulating film. In this method, even if the ink lands at a boundary between the high surface free energy region and the low surface free energy region, the ink moves toward the side of the higher surface free energy due to the difference in surface free energy. As a result, a pattern having a uniform line can be formed, thus enabling the formation of electrode intervals of 5 μm or smaller. However, since the material whose surface free energy changes has a low optical sensitivity, the method requires UV irradiation in amounts greater than 10 J/cm2. The method therefore takes a long irradiation time even with a high-output UV light lamp. As a result, takt time becomes longer and it becomes impossible to simplify the manufacturing process or reduce cost.
Non-Patent Document 1 discloses that a film made of a material whose surface free energy is changed by UV light is layered on a gate insulating film. After a region having a different surface free energy is formed on this film by UV irradiation, as in Patent Document 1, an electrode pattern is formed by an ink jet method. However, the formation of a layered structure prevents simplification of the manufacturing process. Furthermore, as in Patent Document 1, since the material whose surface energy changes has a low optical sensitivity, the method takes an amount of UV irradiation greater than 20 J/cm2.    Patent Document 1: Japanese Laid-Open Patent Application No. 2005-310962    Non-Patent Document 1: Proceedings of the 52nd Spring Meeting of The Japan Society of Applied Physics and Related Societies, p. 1510, 31p-YY-5